The invention relates to a method and to an accordingly-designed device for automatically shutting off and starting an internal combustion engine in a motor vehicle comprising an automated transmission or an automatic transmission according to the preamble of claim 1.
At present, the internal combustion engine is shut off manually by the driver by way of an ignition key in the majority of vehicles available on the market. One exception is a method that is described in DE 100 23 331 A1. Instead of the ignition key, here the brake pedal position or the brake pedal pressure is evaluated. The shut-off process of the internal combustion engine is initiated if the brake pedal is actuated more strongly in the previously actuated position thereof after the motor vehicle has come to a stop. The disadvantage of such methods is that the motor vehicle driver alone is responsible for shutting off the internal combustion engine. Analyses of today's driving behavior show that the internal combustion engine is rarely shut off separately, such as at traffic lights, despite increased environmental awareness and higher fuel prices.
So as to lower fuel consumption and emissions of harmful substances, methods and systems are presently under development, and some of them are already being employed, which automatically shut off the internal combustion engine of a motor vehicle with certain prerequisites or when predefined shut-off conditions exist, and automatically turn the internal combustion engine back on when predefined turn-on conditions exist. Such methods and systems or start-stop devices are suitable in particular for city traffic to reduce fuel consumption since in city traffic the vehicle comes to a halt frequently at traffic lights or due to traffic, and the operation of the internal combustion engine is not necessary during such times.
At present, vehicles comprising automatic transmissions and an automatic start-stop function are already available, in which the engine is automatically shut off from the idle state if the brake pedal remains depressed while the vehicle is stationary and the seat belt is fastened or the driver's door is closed. The automatic start of the internal combustion engine is initiated again as soon as the brake pedal is released. Proceeding from a prior coasting operation of the motor vehicle during which no combustion takes place, prior to shutting down the internal combustion engine, firing of the cylinders is restarted for comfort reasons and to safely reach the idle speed when a re-establishment speed limit has been reached which is greater than the idle speed. This firing (renewed injection and ignition) prior to switching off the internal combustion engine has a modestly negative effect on the fuel consumption and CO2 emission. At the same time, the torque converter lock-up clutch provided between the internal combustion engine and the transmission for power transmission purposes is disengaged no later than when the idle speed is reached so as to prevent stalling of the internal combustion engine.
So as to be able to save even more fuel and further reduce CO2 emissions, systems are already known in which the internal combustion engine is already shut off at higher velocities. For example, DE 24 41 827 C2 discloses a device for switching off and starting an internal combustion engine that already shuts off automatically in coasting operation, which is to say, before the vehicle has come to a stop. A renewed start is only allowed if the rotational speed is considerably lower than the idle speed. The disadvantage here is primarily that the start process cannot be initiated for a very long time.
So as to enable a renewed start at higher rotational speeds or while the vehicle is still moving, a so-called reflex-capable start system is required, since starting by way of a conventional pinion starter is only possible when the engine is stopped. So as to enable an earlier start, a reflex-capable system designed as a starter generator can be used, for example.
Moreover, to shut down the internal combustion engine at higher vehicle speeds, a second battery, which is decoupled from the starter battery, and a suitable decoupling and charging unit (such as DC/DC converter) must be provided, which maintains the vehicle electrical system voltage during a renewed engine start (in particular while the vehicle is still moving) so that safety-relevant operating systems (such as DSC) remain active. The use of a second battery and of a starter generator results in considerable costs that must be borne by the manufacturer or by the buyer.
It is now the object of the invention to provide a method and a corresponding device for automatically shutting off and starting an internal combustion engine, in which shutting off of the internal combustion engine and renewed starting are made possible in a cost-effective manner prior to coming to a stop, including before the engine has come to a complete halt.
This object is achieved by the invention recited in the independent claims. Advantageous refinements will be apparent from the dependent claims.
In principle, the invention is based on a previously-known method for automatically shutting off and starting an internal combustion engine in a motor vehicle comprising an automatic transmission or automated transmission by way of a start-stop device, which initiates an automatic shut-off process of the internal combustion engine during a coasting operation phase of the internal combustion engine, and which initiates an automatic start process of the internal combustion engine when at least one activation command is present and optionally when further activation conditions are met.
The basic idea of the invention is to crank the internal combustion engine during the shut-off process down to very low rotational speed thresholds, and ideally to below a predefined ‘DSC active’ threshold. According to the invention, as a first measure for shutting off the internal combustion engine during a coasting operation phase of the internal combustion engine (during the coasting operation phase, no combustion takes place, which is to say no fuel is injected and no ignition occurs) to at least almost completely suppress a gas exchange in the cylinders of the internal combustion engine during the coasting operation phase using suitable measures. As a second measure, at the same time a torque converter lock-up clutch disposed between the internal combustion engine and the transmission for power transmission purposes is actuated during the shut-off process in such a way that this torque converter lock-up clutch at least almost completely remains engaged, or is engaged. A re-establishment of the combustion process when the so-called re-establishment speed is reached—as is required in the related art—is not necessary, which is to say that the combustion process is suspended until an activation command, and the start process initiated therewith occurs.
This results in consumption advantages not only due to the early shut-off of the internal combustion engine, but also due to the absence of combustion in the interim.
The gas exchange in the cylinders of the internal combustion engine is advantageously suppressed by closing the exhaust valves and/or closing the intake valves of the cylinders of the internal combustion engine. For example, a previously known VVT system (variable valve timing system) having zero-lift capability or switching finger follower may be technology that is used to close at least the exhaust valves. By suppressing the gas exchange in the cylinders, the drag torque is considerably reduced, so that the energy of the vehicle can be converted significantly better into kinetic energy. Moreover, the gas exchange losses are eliminated, as is the gas exchange noise associated with the gas exchange and vibrations that occur in the drive train due to the compression of the cylinder charge.
If in one advantageous embodiment of the invention the gas exchange in the cylinders is carried out or achieved by closing the exhaust valves, the intake valves can advantageously be actuated in such a way that a predefined quantity of fresh air flows into the cylinders and preferably no residual gas remains in the cylinder.
The gas exchange is advantageously suppressed at least almost completely, and the combustion that was suspended due to the coasting operation is maintained, until the internal combustion engine has come to a complete halt (rotational speed=0) and/or until a start process is initiated.
As was already mentioned above, as a second measure during the shut-off process, at the same time a torque converter lock-up clutch disposed between the internal combustion engine and the transmission for power transmission purposes is actuated in such a way that this torque converter lock-up clutch remains or is at least almost completely engaged so as to prevent the gas exchange in the cylinders. The torque converter lock-up clutch is advantageously actuated in such a way that the torque converter lock-up clutch remains at least almost completely engaged until a predefined rotational speed threshold has been reached, or a drop below the same has occurred, wherein the predefined rotational speed threshold is considerably lower than an idle speed threshold. Ideally, the torque converter lock-up clutch is kept engaged down to as low as possible a rotational speed threshold (such as of 300 to 400 rpm). Contrary to existing methods for automatically shutting off the internal combustion engine, the torque converter lock-up clutch is thus engaged or operated with minimal slippage to as low as possible a rotational speed, which is to say that the internal combustion engine remains rigidly coupled to the transmission down to a very low vehicle speed. If minimal slippage of the torque converter lock-up clutch is set, the vibrations of the drive train could also be eliminated at very low rotational speeds.
As a result of the long-lasting frictional connection between the internal combustion engine and the transmission, the internal combustion engine can be restarted during this time at any time on command without delay, and a starter is not required. If the engine is shut down at the lowest possible rotational speeds (approximately 300 rpm), the shut-down period during which no assisted start by way of conventional start systems is possible is considerably reduced.
The initiation of the automatic shut-off process in coasting operation can advantageously be coupled to other conditions. For example, the instantaneous vehicle speed and/or a brake pedal actuation can be evaluated. Ideally, the automatic shut-off process of the internal combustion engine is initiated during the coasting operation phase only if the vehicle speed is lower than a predefined vehicle threshold speed, however which is considerably greater than zero (such as 25 km/h), and/or if the brake pedal is simultaneously actuated during the coasting operation phase.
So as to be able to carry out the method according to the invention, the start-stop device advantageously comprises a control unit for initiating an automatic shut-off process and start process of an internal combustion engine in a motor vehicle comprising an automatic transmission or automated transmission, a first detection device for detecting a coasting operation phase of the motor vehicle, a gas exchange interruption unit for shutting off the gas exchange of the cylinders of the internal combustion engine, and an actuator for actuating a torque converter lock-up clutch disposed between the internal combustion engine and the transmission for power transmission purposes. According to the invention, the control unit is configured in such a way that the same, upon detection of a coasting operation phase of the internal combustion engine, actuates the gas exchange interruption unit in such a way that a gas exchange in the cylinders of the internal combustion engine is at least almost completely suppressed. At the same time, the actuator for actuating the torque converter lock-up clutch is actuated in such a way that the torque converter lock-up clutch remains or is at least almost completely engaged.
The gas exchange interruption unit is advantageously configured in such a way that the same at least almost completely closes the exhaust valves and/or the intake valves of the cylinders of the internal combustion engine so as to suppress the gas exchange. Ideally, a VVT system ((fully) variable valve timing system) having zero-lift capability or switching finger follower may be used for this purpose. Since the ability to actuate the torque converter lock-up clutch must also be ensured at low transmission speeds, the actuator for actuating the torque converter lock-up clutch that is disposed between the internal combustion engine and the transmission for power transmission purposes is advantageously an electrical or switchable hydraulic transmission fluid pump or a hydraulic pressure accumulator.
The method according to the invention will be described again in more detail based on the following exemplary embodiment. In the drawings:
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.